#include<iostream>
using namespace std;
class Nguoi
{
public:
	virtual void Chao(){
		cout << "Toi chua biet chao";
	}
};
class NguoiViet : public Nguoi
{
public:
	void Chao();
};
class NguoiAnh : public Nguoi {
public:
	void Chao() {
		cout << "Hello.";
	}
};
